This invention relates to a process, catalyst, and a method for making a catalyst for the selective hydrotreating of hydrocarbons. More particularly, this invention relates to a process, catalyst, and method for making a catalyst for the selective hydrotreating of cracked naphtha using a catalyst comprising a Group VIB metal component, a Group VIII metal component, a magnesium component, and an alkali metal component.
Cracked naphtha is a blending component commonly used in refinery gasoline pools containing both sulfur and olefinic compounds. Cracked naphtha can be produced in refinery fluid catalytic cracking processes, coking processes, or hydrocracking processes, among others, where a gasoline boiling range component is generated or distilled having olefinic compounds. The sulfur, which can be present in cracked naphtha in amounts ranging up to 1.0 percent by weight, is both a potential air pollutant and a poison to the catalysts used in certain automobile catalytic converters. The olefinic compounds, which can be present in cracked naphtha in an amount ranging up to 60 percent by weight, have octane numbers that are generally higher than those of their corresponding saturates, and as such, it is generally undesirable to saturate olefins to saturates wherein the component stream is to be blended directly to gasoline.
Sulfur dioxide that is generated by the burning of high sulfur fuels (in the gasoline, distillate, and residual boiling point range) has been identified as one of the chief air pollutants. Hydrotreating is an important method for producing fuels with relatively low sulfur concentrations and commercial hydrotreating plants for treating fuel oils are now, and have been in operation to provide fuel oils that have legally acceptable sulfur levels. However, the Clean Air Act of 1990 has now mandated that Volatile Organic Compound (VOC) and Toxic Emissions be reduced by an additional 15% by the year 1995 and 25% by the year 2000, in ozone nonattainment areas, through fuel reformulation. It is likely that a considerable portion of this reduction will be performed through further reduction in the sulfur content of gasoline. Since gasoline production can comprise in excess of 50 wt % of the production output of a refinery, refinery hydrotreating loads can be expected to increase dramatically.
Therefore, if the cracked naphtha is to be desulfurized without eliminating or seriously reducing the amount of olefins that are present therein, the hydrotreating process that is used must be very selective, i.e., capable of removing substantially all of the sulfur without severely saturating the olefins that are present. Currently, there are several hydrotreating catalysts and processes that find considerable use in the petroleum refining industry. Such hydrotreating catalysts include cobalt and molybdenum and their compounds on a suitable support, cobalt, molybdenum, and nickel on a suitable support, nickel and tungsten and compounds thereof on a suitable support, and nickel and molybdenum and compounds thereof on a suitable support. The support, in general, is a weakly-acidic catalytically active alumina. Such conventional hydrotreating catalysts are less selective and not only remove sulfur from the petroleum hydrocarbon stream being treated, but also tend to saturate olefins, reducing the octane of the petroleum hydrocarbon stream.
Selective hydrotreating has been the subject of several U.S. patents, each meeting with varying degrees of success and limitations.
U.S. Pat. No. 4,334,982 to Jacquin et al. discloses a process for hydrotreating hydrocarbon streams having high olefin content with a low surface area hydrotreating catalyst comprising cobalt and tungsten or molybdenum. The total weight of cobalt, tungsten, and molybdenum, expressed as oxides, is from 2% to 30% by weight of the total hydrotreating catalyst, and the atomic ratio of cobalt to cobalt, molybdenum, and tungsten is greater than 0.55. The catalyst does not contain, nor does the patent teach or suggest use of a magnesium component.
U.S. Pat. No. 4,140,626 to Bertolacini et al. discloses a process for the hydrotreating of cracked naphtha with a catalyst comprising Group VIB and Group VIII metals deposed on a support comprising at least 70% by weight magnesium oxide. While magnesium oxide supported catalysts are effective hydrotreating catalysts, catalysts having a substantial amount of magnesium oxide can be relatively soft and have a low surface area. Hydrotreating catalysts having relatively low crushing strength and that incur high abrasion losses are less durable and generally less attractive for use in petroleum refinery operations.
U.S. Pat. No. 3,957,625 to Orkin discloses a process for the hydrotreating of a cracked naphtha stream wherein the cracked naphtha is split into low and high boiling naphtha fractions. The high boiling fraction is contacted with a low surface area hydrotreating catalyst comprising a cobalt-molybdenum-alumina catalyst having a promoter component consisting of barium, magnesium, cadmium, or a rare earth metal. The catalyst does not contain, nor does the patent teach or suggest use of an alkali metal component. The hydrotreated high boiling naphtha fraction is subsequently recombined with the low boiling naphtha fraction, and a gasoline blending component having a lower sulfur content is produced.
It has now been found that combining a Group VIB and Group VIII metal with a non-acidic support having a magnesium component and an alkali metal, in the manner described in the present invention, can provide superior catalyst and process performance. This is believed to be true since cracked naphtha hydrotreating selectivity appears to correlate inversely to catalyst acidity. While it is known that large amounts of a magnesium component can increase basicity, the resultant loss in catalyst durability can make the use of a catalyst having large concentrations of magnesium economically and operationally impracticable. It has now been found that the magnesium concentration in a selective hydrotreating catalyst can be substantially reduced when combined with an alkali metal in the manner described in the present invention. Moreover, it has also been found that a selective hydrotreating catalyst with both magnesium and an alkali metal provides superior performance to catalysts having either component separately.
It has also been found that the above described catalyst and processes utilizing the same can be further enhanced by the particular method of making the selective hydrotreating catalyst. Impregnation of the support with the Group VIB and Group VIII metals and coextrusion prior to post impregnation of the catalyst with the magnesium component and the alkali metal provides superior performance to other methods of making the catalyst of the present invention and to prior art catalysts in general.
It is therefore an object of the present invention to provide a process and catalyst that provide substantial cracked naphtha desulfurization while minimizing subsequent octane reduction.
It is another object of the present invention to provide a process and catalyst that reduce cracked naphtha diene concentration.
It is another object of the present invention to provide a process and catalyst that improve gasoline color and stability.
It is another object of the present invention to provide a catalyst that has superior crush strength and resists abrasion losses.
It is yet another object of the present invention to provide a method of making a catalyst and a process for utilizing a catalyst produced by this method that further enhance selective hydrotreating performance.
Other objects appear hereinafter.